1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal composition and a light switching element making use of the ferroelectric liquid crystal composition. More specifically, this invention relates to a ferroelectric chiral smectic liquid crystal composition having a long helical pitch and a light switching element having a quick response property.
2. Description of the Prior Art
Liquid crystal compounds are widely used as display materials but most of such liquid crystal display elements are of a TN display mode, in which a liquid crystal material belonging to a nematic phase is used. Since the TN display mode is of a non-emissive type, it has characteristic features that it does not give fatigue to eyes, and works with extremely small power consumption. On the other hand, it has such defects that the response is slow, and that display may not be seen depending upon the angle from which it is viewed. Recently, an attempt to improve the liquid crystal material has been made for obviating these defects, particularly for meeting a demand for a quick response of display device. However, the TN display mode still involves a problem of a large response time lag to be solved, as compared with other displays of an emissive type [e.g. EL (electroluminescence) display, plasma display, etc.]. For making much of the characteristic features of the liquid crystal display element such as the passive type and small power consumption and securing a response property comparable to those of the emissive display, it is indispensable to develop a new liquid crystal display mode which can be substituted for the TN type display mode. In one of attempts of such development, N. A. Clark and S. T. Lagerwall proposed a display device which utilizes the light switching phenomenon of a ferroelectric liquid crystal [see N. A. Clark, S. T. Lagerwall; Appl. Phys. Lett., 36, 899 (1980)]. The existence of a ferroelectric liquid crystal was disclosed for the first time by R. B. Meyer et al. in 1975 [see R. B. Meyer et al.; J. de Physique, 36, L-69 (1975)]. From the viewpoint of the liquid crystal structure, ferroelectric liquid crystals belong to the chiral smectic C phase, the chiral smectic I phase, the chiral smectic F phase, the chiral smectic G phase, and the chiral smectic H phase (which will be hereinafter abbreviated as the S.sub.c * phase, the S.sub.I * phase, the S.sub.F * phase, the S.sub.G * phase, and the S.sub.H * phase, respectively).
Application of the light switching effect of the S.sub.c * phase to display elements provides three characteristic features superior to those of the TN display mode. The first feature is a very quick response The response time is 1/100 or less than that of a common TN display mode. The second feature is that it provides a memory effect which makes multiplexing drive easy in cooperation with the above-mentioned quick response property. The third feature is an easiness in attaining the gray scale or gradation of brightness. In the case of the TN display mode, since the gray scale is attained by controlling the applied voltage, difficult problems involving the dependence of threshold voltage on temperature and the dependence of response time on voltage are encountered. By contrast, in the case of application of the light switching effect of the S.sub.c * phase, the gray scale can be attained easily by controlling the polarity-inversion time. Thus the latter is very suitable for graphic display or the like.
As display methods, two methods can be considered. One of them is a birefringence type which uses two polarizer plates, and the other is a guest-host type which uses dichroic dyes. Since the S.sub.c * phase has spontaneous electric polarization, molecules turn over by a .pi. rotation around helical axes thereof as the axes of rotation by inverting the polarity of applied voltage. By filling a liquid crystal composition having a S.sub.c * phase into a liquid crystal display cell that has been subjected to a surface treatment for aligning liquid crystal molecules parallel to electrode surface, disposing the liquid crystal cell between two polarizers so arranged in advance that the polarization plane of one of the polarizers is parallel to the director of liquid crystal molecules, and inverting the polarity of applied voltage, a bright range of vision and a dark range of vision (which are determined by the angle between the polarization planes) can be inter-exchanged. On the other hand, when the display is operated by guest-host mode, a colored range of vision and a colorless range of vision (which are determined by the arrangement of polarization plates) can be inter-exchanged by inverting the polarity of applied voltage.
Ferroelectric chiral smectic liquid crystal compounds known today are exemplified in Table 1 [see J. Physique, 37, C3-129 (1976)]. As can be understood from Table 1, most of ferroelectric liquid crystal compounds have a temperature range showing ferroelectricity, namely a temperature range showing the S.sub.c *, S.sub.I *, S.sub.F *, S.sub.G * or S.sub.H * phase, above room temperature. Thus they cannot be employed as display element materials as they are. Besides, chiral smectic liquid crystal compounds have a possibility that there may exist the smectic B phase or the smectic E phase (which will be hereinafter abbreviated as "S.sub.B phase" and "S.sub.E phase", respectively) showing no ferroelectricity. Therefore, a chiral smectic liquid crystal composition showing ferroelectricity in a practical temperature range cannot be easily obtained. No chiral smectic liquid crystal compositions (a) showing ferroelectricity in a practical temperature range including room temperature, (b) having a large spontaneous electric polarization, and (c) having a long helical pitch have been obtained as yet. TBL3 TABLE 1 Compound Phase transition temperature ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ##STR7## ##STR8## (Note) In the column of "phase transition temperature", C, S.sub.A and I stand for crystal, smectic A, and isotropic liquid phases, respectively, and th others stand for the respective phases as mentioned hereinbefore.